Our electric furnace has been acting strangely and I suspect it's a bad sequencer. It will run and run and not shut off (came home yesterday and it was 86 degrees F in the house). I shut it off and now it seems to be operating normally, though it seems to be running more often than it should. When the furnace is not running I put a volt meter across the heater coil terminals and I'm reading 240v on the number 3 & 4 elements, 0v on numbers 1 & 2. I'm thinking that if it's not running there should be 0 across the elements.

just to clarify terminology, when you say you are checking elements, i'm visualizing four elements and you are testing across the terminals of each and reading 240 across two of them and 0 across the other two. in that case, a potential read of 240 indicates a sequencer problem. if you are referring to sequencer terminal numbers, houston204 is right. potential across a switch in an otherwise intact circuit indicates that switch is open. in that case, with no call for heat, the sequencer is ok.

you said the house was getting too hot. if it happens again, check the low voltage connections on the sequencer while it is overshooting and verify whether or not a 24 volt signal is present. if it is, then you could have a thermostat problem.